1. Field of the Invention
The present invention relates to polymeric foam comprising styrene-acrylonitrile copolymer and a process for preparing such polymeric foam.
2. Description of Related Art
Polymeric foam is useful for many applications including thermal insulation and floral and craft applications. Unlike most thermal insulation applications, most floral and craft foam is desirably white in color. Moreover, floral and craft foam desirably has an average cell size on the order of millimeters in diameter as opposed to thermally insulating foam that benefits from average cell diameters more like 250 microns or less. Large cell sized foam is more difficult to prepare with a white appearance. Small cell diameter foam, such as thermal insulating foam, scatters light more readily than large cell sized floral and craft foam. Scattered light tends to increase a white appearance and can mask discoloration in the polymer matrix of foam. The large diameter cells of floral and craft foam do not scatter light as well so the color of the polymer matrix is more apparent. The large cell size of floral and craft foam makes it particularly challenging to prepare with a white appearance.
Styrene-acrylonitrile (SAN) copolymer has become an increasingly desirable polymer for preparing polymeric foam because it has higher water solubility than polystyrene homopolymer, which means environmentally friendly aqueous blowing agents can be used more readily with SAN copolymer than polystyrene homopolymer. Unfortunately, SAN copolymers are also more susceptible to yellowing than polystyrene homopolymers. Yellowing promotes a sense of impurity, age and degradation in otherwise white foam. This is particularly problematic for floral and craft foam where an especially white foam appearance is necessary. Even minor yellowing becomes noticeable in the large cell sized foam matrix of floral and craft foam. That makes using SAN copolymer particularly challenging for floral and craft applications even though use of an aqueous blowing agent is desirable.
The reason why SAN copolymer yellows more readily than polystyrene homopolymer is not fully understood. One study reports that the yellowing in SAN copolymers is at least partly the result of cyclized AN sequences forming in the copolymer backbone as well as possibly small-molecule (oligomer) residues in the polymer. (see, Macromolecules, Vol. 26, No. 22 (1993)). The same article does not address a solution for avoiding oligomer formation, but does offer suggestions on how to modify its continuous stirred tank reactor (CSTR) process to possibly reduce cyclized AN sequences.
United States patent application 2008/0300333A1 discloses a method for preparing copolymers comprising a combination of three monomers (vinyl aromatic monomer, vinyl cyanides and dicarboxylic anhydrides) and thermoplastic articles made from the copolymers. An objective of these copolymers is to achieve vinyl aromatic copolymers having less intrinsic color.
United States patent application 2001/0007890A1 discloses a styrenic copolymer polymerized in a presence of antioxidants to produce a copolymer having a low level of intrinsic yellow color.
It is desirable to be able to produce a white floral and craft foam of SAN copolymer without having to include dicarboxylic anhydride monomers or polymerize the SAN copolymer in the presence of antioxidants. It would be even more desirable to produce floral and craft foam of SAN copolymer that has less yellowing than achievable only by adding antioxidants.